feat(provider): version provider observation metadata with schemaVersion

OpenAI and xAI now stamp provider_observation_json with schemaVersion: 1 so
historical records can be decoded per version (missing version = v0). Per
project-redundancy-boundary-audit DB-03.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/crates/providers/openai/src/provider.rs

@@ -1127,6 +1127,8 @@ impl OpenAiResponseObservationState {
 
     fn provider_metadata(&self) -> Option<ProviderResponseMetadata> {
         let mut metadata = Map::new();
+        // 观测对象版本信封：读取端按 schemaVersion 解码，缺失视为 v0（见 DB-03）。
+        metadata.insert("schemaVersion".to_owned(), json!(1));
         let effective_model = self
             .response_metadata
             .effective_model

backend/crates/providers/xai/src/provider.rs

@@ -1543,6 +1543,8 @@ fn xai_transport_metadata(
     if metadata.is_empty() {
         return None;
     }
+    // 观测对象版本信封：读取端按 schemaVersion 解码，缺失视为 v0（见 DB-03）。
+    metadata.insert("schemaVersion".to_owned(), Value::from(1));
     ProviderResponseMetadata::new(serde_json::to_string(&Value::Object(metadata)).ok()?)
 }
 

backend/crates/providers/xai/tests/provider/contract.rs

@@ -1012,6 +1012,7 @@ async fn execute_returns_cold_stream_and_records_selected_account() {
     assert_eq!(
         metadata,
         json!({
+            "schemaVersion": 1,
             "clientCache": "miss",
             "dnsSource": "system",
             "dnsMs": 7,